1. Field of the Invention
This invention relates to a fluid catalytic cracking (FCC) process for converting petroleum derived hydrocarbon feedstocks to liquid boiling range products. The invention more particularly relates to a method of combining feedstock with catalyst to improve flow characteristics in a fluid catalytic (FCC) riser reactor.
2. Other Related Methods In the Field
Fluid catalytic cracking processes are well known in the art. In the catalytic cracking process, liquid hydrocarbon feedstocks such as diesel, gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil and combinations thereof are contacted with active cracking catalysts such as a crystalline alumina silicate at temperatures of from about 800.degree. F. to 1400.degree. F., preferably 850.degree. F. to about 1050.degree. F. at pressures of about 20 psia to 45 psia for a period of time in the order of about 10 seconds or less, preferably about 0.5 seconds to convert the feedstock. The process utilizes pelleted catalyst in a moving bed, or powdered or microspherical catalyst in a fluidized reaction zone. There are many types of cracking catalysts. One group referred to as zeolite catalysts, are commercially preferred. Higher boiling hydrocarbons such as vacuum gas oil are cracked using zeolite catalyst to produce lower boiling hydrocarbons with the relative product mix, including C.sub.4 olefins, gasoline and solid carbonaceous deposits determined by catalyst contact time, reactor conditions and feedstock composition.
The vaporization rate of liquid hydrocarbon feedstock is significant in a fluidized catalytic cracking reaction zone. With liquid feedstock, the time required for vaporization of atomized liquid droplets reduces the time available for the desired catalytic reactions thus tending to reduce feed conversion per pass with a consequent adverse influence on yield. The process can be carried out and good product yields achieved with an all vapor feedstock because the desired reactions occur in the vapor phase. Vapor phase also provides rapid transport of reactants to and from active catalytic sites which favors a high reaction rate. However, preheating the feedstock for complete vaporization prior to injection into the reactor adversely affects process economics.
Thus, it has been found that vaporization of the feedstock is critical. Essentially complete vaporization of atomized feedstock must be effected in the injection zone.
It has also been found advantageous to vaporize all feedstock in about one second or less by atomizing the liquid to form a fine dispersion of droplets of about 350 microns or less in diameter. With such a fine dispersion, all the droplets do not have to come in direct contact with the catalyst heat source for rapid vaporization. Heat flows rapidly by radiation and thermal conduction through vapor and evaporates the liquid droplets without direct contact with the catalyst.
A number of atomization nozzles have been used commercially to vaporize feedstock. The nozzle described in U.S. Pat. No. 2,747,936 to F. W. Wahlin is typical of impact plate and orifice nozzles used in the industry.
Atomization of liquid fuels is discussed in Perry's Chemical Engineer's Handbook, 4th ed., pp 9-24 to 9-27. Atomization of liquid fuels is subdivided into external atomization, internal atomization and mechanical atomization.
In external atomization, oil is atomized by hitting a flowing oil stream with a perpendicular flow of an atomizing fluid. Steam may be used as the atomizing fluid in an amount of 0.1 to 8 pounds of steam per pound of oil. Internal atomization is accomplished by mixing steam and oil together before flowing through an atomizer venturi.
Mechanical atomization is accomplished in its simplest application by forcing the oil under pressure through tangential slots in a sprayer plate to impart a rotating motion. The droplets leave the nozzle through an orifice. Droplet size is determined by the pressure drop across the sprayer plate.
Pressure drop is the primary parameter for control of droplet size in atomizers. Atomizers have been used which apply a pressure drop of 30 to 50 psi, yielding 400 to 500 micron droplets. Spiral tip atomizers have been used, e.g. U.S. Pat. 2,518,116 to J. U. Bete and U.S. Pat. No. 4,514,291 to P. E. McGarry et al. Those spiral tip nozzles apply pressure drops of 40 to 50 psi to yield 200 to 300 micron droplets.
There is a need in the art for a method to vaporize feedstock in a fluid catalytic cracking (FCC) process to achieve transport of both feedstock and catalyst together up the riser reactor in plug flow.